The device-to-device proximity service (Device to Device Proximity Service, D2D ProSe for short) has become a research topic of the Long Term Evolution (Long Term Evolution, LTE) system Rel.12 of the 3rd Generation Partnership Project (3rd Generation Partnership Project, 3GPP for short). The D2D Prose provided by using an LTE physical layer can not only expand a service scope of the LTE system but also enable the D2D Prose to be used by more users. The ProSe includes D2D discovery and D2D communication. D2D discovery refers to that user equipment with D2D function discovers a signal and learns of existence of anther user equipment, and D2D communication refers to direct communication between devices with D2D functions. Time synchronization (including frame synchronization, bit synchronization, sampling point synchronization, and the like) and frequency synchronization of the system are two major issues during design of D2D communication. Only after obtaining precise synchronization can a receiving device ensure that subsequent communication functions such as channel estimation, demodulation, and decoding work properly.
In the prior art, synchronization of a D2D communications system is as follows: A transmit end sends a synchronization sequence, and a receive end acquires system synchronization by performing a time synchronization algorithm and a frequency synchronization algorithm on the synchronization sequence. When the transmit end sends the synchronization sequence, a time reference source is required for calibrating a synchronization signal sent by the transmit end. Synchronization precision of different time reference sources is different, which leads to different precision of synchronization signals sent by the transmit end. As a receiver, user equipment usually determines a synchronization signal according to strength of the synchronization signal. This determining manner is relatively monotonous and lacks flexibility.